The start of the mission FRONTLINE
is your best bet here. There is a rail car with about 6 soldiers on it. What I did was kill the commander which would trigger the other soldiers to jump out of the cart. This is where I would throw a normal grenade on either side which blew up most enemies. After that I would reload checkpoint as it doesn't reset your progress.
If you don't have any grenades then you can load up the level right before it called ARMORY
where you can buy them from one of the vendors.
After this achievement you can switch to fire grenades to unlock: